Stable gas-generating composition



June 1960 T. A. BURGWALD ET Al. 2,942,964

STABLE GAS-GENERATING COMPOSITION Filed April 26, 1955 \A A. l/ r F lg.2

Fig. 1

12L Y Theodgggfifiggg wald Edwin E. More/l0 INVENTORS:

2,942,964 STABLE GAS-GENERATING COMPOSITION Theodore A. Burgwald,Whiting, and Jack Linsk, Highland, Ind., and Edwin F. Morello, Juliet,111., assignors to Standard Oil Company, Chicago,glll., a corporation ofIndiana l i 7 Filed Apr. 26, 1955, Ser. No. 504,048

8-Claims. c1.s2-.s

This invention relates to a composition for the generation of a gas. Theinvention pertains more particularly to an improved gas-generatingcomposition consisting essentially of ammonium nitrate as the primarygas-producing component of the composition, intimately mixed with acombustible bindermaterial. The composition also contains an inorganiccombustion catalyst, a synergist to accelerate the burning rate of thecomposition and other components to stabilize thermally the graincomposition; Still more particularly, the invention relates to acomposition for thefgeneration of gasby'the combus- :tion of a shapedgrain consisting essentially of the ammonium nitrate, a combustioncatalyst, a combustible bindery-a carbon synergist to accelerate burningrate and additives to stabilize the grain ballistically when storedunder: high temperature conditions. Such composition is useful for thepropulsion of rockets for ground-to-ground missiles,ship-to-shore'missiles, air-to-air missiles and airto-ground missilesand the composition may also be used as'propulsionmeans in assisttake-off in military and commercial aircraft. Grains made from thecomposition can be stored for long periods of time at relatively highatmospheric temperatures without damage to the ballistic properties ofthe grains.

Ammonium nitrate iswidely used as a component of high explosives,particularly the so-called safe explosives. Even though ammonium nitrateis classified as a high explosive, it is extremely insensitive toordinary heating and toshock and cannot be readily detonated by thelocal application of heat or by a blasting cap. Further, when ignited,ammonium nitrate alone does not burn uniformly and has a tendency to goout. prove the burning quality, to increase the sensitivity, and toutilize the excess free oxygen available from the decomposition of theammonium nitrate and to provide shaped grains suitable for use in rocketmotors and assist take-off motors, combustible binder material is usedin the ammonium nitrate composition. a

The use of ammonium nitrate-base compositions as solid propellants forrockets and assist take-01f units is attractive because of the 10W costand-availability of the ammonium nitrate, because of the relatively lowflame temperature of decomposition, of the nitrate, that is, be-- tween3150" and 2900 F. and because the excess free oxygen available from thedecomposition permits the use of oxidizable material to increase theenergy available from the decomposition. However, the physicalcharacteristics of ammonium nitrate and grain material producedtherefrom introduces problems with respect to shaped grains. Thusammonium nitrate exists in diiferent crystalline forms, the transitionfrom one form to a different form involving a volume change of theammonium nitrate. Volume changes which occur at about 90 F. and also atabout F. involve 3.5% and about 3% increase respectively. 'It is,therefore, obvious that an ammonium nitrate base composition could beseriously affected by storage where large volume changes occur attemperatures common to the storage conditions. One

the principal gas-generating material.

In order to im- 2,942,964 Patented June 28, 1950 requirement for solidpropellant suitable for military use is that it'be ballistically stableafter prolonged storage at temperatures as high as 170 F. or followingstorage at temperatures as low as F.

Another requirement is that the grain fabricated from the compositionwill not shatter as a result of being subjected to alternate high andlow temperatures, i.e., 170 F. followed immediately by a temperature of-75 F. in a series of at least two repeated cycles and also that thegram must burn uniformly following such alternate high and lowtemperature treatment. Still another requirement of the grain materialis that it may be ignited at extremely low temperatures, i.e., -75 F.and temperatures as high as to F. as Well as at intermediate ambienttemperatures following hot or cold storage.

That is, a grain composition must be reliable with respect to firingafter being subjected to variable temperature environment. Anotherrequirement is that the grain material does not gas appreciably whensubjected to a temperature encountered in service. tive of chemicalchange in the composition and it is usually necessary to use a gassinginhibitor to meet the requirement of low gassing properties. i

An object of this invention is the preparation of a gas-generatingcomposition using ammonium nitrate as Another object of the invention isto produce a gasgenerating composition suitablefor use in rockets andassist take-off units, which composition is ballistically stable afterbeing subjected to high and low temperature environment.

' Still another object of the invention is to produce a gas-generatingcomposition which is relatively stable chemically at temperaturesencountered in service.

Yet another object of the invention is to produce a gasgeneratingpropellant composition which exhibits good ballistic performancefollowing storag at high temperatures. Other objects will be apparent asthe detailed description of the invention proceeds.

The composition of this invention consists essentially (1) At least 70%by weight of ammonium nitrate, usually from about 72% to, 76%. g

(2) From about 1% to about 6% by weight of an inorganic combustioncatalyst consisting essentially of a mixture of ammonium dichromate andinsoluble Prussian blue;

(3) Between about l8%-and 27% by weightof a combustiblebinder, whichbinder consists essentially of:

(a) From about 18% to' about 35% by weight of cellulose acetate whichanalyzes between about 51% and 57% by Weight of acetic acid; a

(b)' Between about 20% and 40% by weight of the liquid polyestercondensation product of ethylene glycol and diglycol-ic acid, saidcondensation prodnot having a molecular weight within therange of fromabout 250 to about 1000; and a a a (0) Between about 20% and 40% byweight of a nitrodiphenyl ether; I

(4) Between about 0.3% and 3% by weight of an amine gassing inhibitor,based on the total weight of the gram;

(5) From about 0.3% to about 3% by weight of finely divided'carbon i.e.,carbon black or finely divided activated carbon; and

(6) From about 0.05% to about 0.5% by weight based on the total weightof the grain of a non-ionic surfactant material selected from the classconsisting of polyoxyalkylene glycol-fatty acid esters having molecularweights of about 1000 to about 2000, polyoxypropylenepolyoxyethyleneglycol having molecular weights of 2000 Gassing rate is indicato about3300 and a sorbitan oleate containing from 1 to 3 oleate radicals.

The term ammonium nitrate as used in the specification and claims isintended to mean either ordinary commercial grade ammonium nitrate whichmay or may not contain small amounts of impurities. The nitrate may becoated with a small amount of anti-caking agent such as petrolatum orparaffin wax. It also includes military grade ammonium nitrate as wellas mixtures of minor amounts (usually less than of other inorganicnitrates such as sodium nitrate or potassium nitrate with the ammoniumnitrate. .A mixture of finely ground and unground or coarsely groundammonium nitrate is beneficial to the ability of solid propellant grainsto pass cycling tests described hereinbelow. Usually the majorproportion of the ammonium nitrate is ground, a minor proportion i.e.,up to about 25% or 30% being unground. Finely ground nitrate is desiredin order to 'attain maximum grain density. The ammonium nitrate is ofsuch particle size that at least about 50% will pass through a #100 U.S.Standard sieve and at least 90% will pass through a #20 US. Standardsieve.

The inorganic combustion catalyst of this invention consists of amixture of ammonium dichromate and insoluble Prussian blue catalyst. Theinsoluble Prussian blue is a more effective catalyst at high pressurethan soluble Prussian blue. Thus when operating the combustion chambercontaining the solid propellant at pressures between about 500 and 2000p.s.i., a higher burning rate is obtainable when using the insolublePrussian blue catalyst than is obtainable when using soluble Prussianblue as catalyst for an otherwise similar composition. In gen.- eral,the ratio of ammonium dichromate to Prussian blue of the compositionshould vary Within the range of from about 1 to 3 and 3 to 1 andpreferably about two parts by weight of ammonium dichromate catalyst perpart by Weight of insoluble Prussian blue catalyst in the composition.The total amount of inorganic combustion catalyst will usually be withinthe range of from about 1% to 6% by weight of the grain composition, andprefer-ably about 3% by weight.

The binder in the composition of this invention consists essentially ofcellulose acetate plasticized with a mixture of a nitrodiphenyl etherand the condensation product of a dihydric alcohol with a dicarboxylicacid such as ethylene glycol with diglycolic acid. An amine inhibitor isadded to the binder material.

Cellulose acetate is used as the polymeric material to be plasticized toform binder material. It is known as a partially esterified material andis described as having an acetic acid content between about 51% and 57%by weight. The term percent by weight acetic acid denotes the amount ofacetic acid obtained upon saponification of the cellulose acetate and isexpressed as percent of the initial material. A particularly suitablecellulose acetate is one which analyzes between 54% and 56% by Weight ofacetic acid. Lacquer grade cellulose acetate is a particularly goodpolymeric material for incorporation in the grain composition. Lacquergrade cellulose acetate is described in addition to its acetic acidcontent by its viscosity when dissolved in acetone, the viscosityranging from 2 to 80 centipoises at 25 C. for a 20% solution in acetone.

The preferred cellulose acetate of this invention analyzes between about54 and 56% by weight of acetic acid and has a viscosity between about 2and 10 centipoises. A binder having the proper characteristics toprovide a shaped explosive with the ammonium nitrate contains betweenabout 18% and by weight of the defined cellulose acetate. Preferably thebinder contains about 33% cellulose acetate. The complete graincomposition contains from about 6% to about 9% by weight of thecellulose acetate.

The preferred plasticizerutilized in the binder consists essentially of2 components, that is, V

(a) The product of the polyesterification of a dihydric alcohol such asethylene glycol with a dicarboxylic acid such as diglycolic acid, amolar excess of the alcohol being used in producing thepolyesterification product; and

(b) A dinitrodiphenyl ether such as 2,4-dinitrodiphenyl ether.

In order to obtain a polyesterification reaction product satisfactory asa plasticizer component, a polyester having substantially nocross-linkages is desired. It has been discovered that the dihydricalcohol used in the preparation of the plasticizer must be selected fromat least one of the dihydric alcohols in the class consisting ofethylene glycol, polyethylene glycol, propylene glycol, polypropyleneglycol and in general, the polyglycols must have a molecular weight lessthan about 400 in order to produce a polyester of the desiredproperties.

The dicarboxylic acids used in the preparation of a plasticizer must beselected from at least one of the class consisting of aliphaticdicarboxylic acids and aliphatic oxydicarboxylic acids, which acids havebetween 2 and 6 carbon atoms in the molecule. Examples of dicarboxylicacids are malonic acid, succinic acid, glutaric acid and adipic acid.Examples of the oxydicarboxylic acids are diglycolic acid (a,ot'oxydiacetic acid) and oxydipropanoic acid.

The condensation product of ethylene glycol with diglycolic aciddescribed in the copending application of Norman J. Bowman and Wayne A.Proell, entitled Polyester Plasticizer, filed November 30, 1954, SerialNo. 471,992, now abandoned, is preferred. It has been discovered thatthe molecular weight of the productof the polyest'erification reactionhas a considerable effect on the plasticizing properties of the product.A relatively low molecular weight, that is, Within the range of about250 to about 600, is desirable and is obtained by using a molar excessof alcohol over acid. The molar ratio of alcohol to acid should bebetween about 1.02 and 1.3, preferably not less than about 1.15 toobtain the most satisfactory condensation product, with respect toplasticizer properties. The binder of this invention usually containsfrom 20% to 40% by weight of the condensation product based on the totalweight of the cellulose acetate, dinitrodiphenyl ether and thecondensation product i.e., exclusive of the amine inhibitor in thebinder. The com plete grain composition contains from about 5% to about9% by weight of the condensation product.

The second plasticizer component utilized in the preparation of thebinder of this invention is a nitrodiphenyl ether, preferably2,4-dinitrodiphenyl ether which may be prepared in high purity and highyield by reacting 2,4- dinitrochlorbenzene with phenol in aqueouscaustic medium, as taught in copending application of Wayne A. Proelland Norman J. Bowman, entitled Thermoplastic Compositions, filed October27, 1954, Serial No. 465,132. Other nitrodiphenyl others may be used butin general the average nitro content is not greater than about 2.5 nitrogroups per molecule and not more than about 2 nitro groups substitutedon a given phenyl group of the molecule. Dowtherrn A, a commercialproduct containing about 73% by weight of diphenyl ether, the remaining27% being all diphenyl may be nitrated to contain such distribution ofnitro groups and the product may be used as the nitrodiphenyl etherplasticizer component. However, 2,4-dinitrodiphenyl ether prepared from2,4-dinitrochlorbenzene is preferred. The complete grain formulationcontains from about 6% to about 9% by weight of the nitrodiphenyl etherplasticizer component.

Certain aromatic amines when introduced into the ammonium nitrate-basedgrain catalyzed by Prussian blue and ammonium dic'nromate catalyst, havethe very desirable effect of decreasing the gassing of the grain atrelatively high temperature. Different classes of amines may be used asgassing inhibitors. However, diphenylamine is preferred. A markedimprovement in the gassing en istees g tendency offthe compositionisobtained by the use of relatively small amounts of the amine, i.e.,from about 0.5% to 3% by weight based on the total weight of. the

grain and preferably from about 1% to about 3 of the a'mine'base'd onthe total weight-of the grain. f

'iThe carbon component, which is added as a synergist to increase theburningrate of the composition, may be finely divided highly adsorptive'carbons" such as Nor'iti and Nuchar. However, it is preferred to usecarbon naces.

The carbon blacksare characterized by low ash. content and by havingextremely small particle size, that is, 50 to 5000 A., and they containadsorbed hydrogen and oxygen. The Bead type carbon blacks may be used.

Examples of bead type carbon blacks suitable as catalyst synergists areMicronex Beads (channel blacks) and StatexBeads (furnace blacks). Thecarbon component of the propellant composition of this inventioncontains not more than 5% ash and preferably less than 0.5% ash. Fromabout 0.3% to 3% .byweight of carbon black may be used in the graincomposition of this in-' vention. f

It has been discovered that certain non-ionic surfactants (sorbitans)derived from sorbitol, the sorbitan oleates being mixtures containing anaverage of from 1 to 3 oleyl radicals foreach molecule of anhydrizedsorbitol. Sorbitol is dehydrated to give condensed link structures of'sorbitans which, when esterified with oleic acid, produce the preferredSpans or Arlacels. Arlacel C, which issorbitan sesquioleate, has beenfound to be particularly' effective as an additive to promote ballisticdependability of the'grain compositions. This material is an oily liquidat C. having a specific gravity of 0.95-1.00, a flash point of about 450F. and a fire point of about 530 F.

Arlacel C has a viscosity at 25 C. of 900-1100 centipoises. Another veryeifective non-ionic surfactant isSpan 8S, which is a sorbitan trioleatehaving a viscosity at 25 C. of 100250 centipoises. It has a specificgravity of 0.92-0.98, a flash point of 500 and a fire point of 570. F.

Another type of non-ionic surfactants which we have found to beeifective in stabilizing the compositions are polyalkylene oxidecondensation products and the ester derivatives thereof. These are knownas non-ionic block polymers and are prepared by condensing propyleneoxide in the presence ofmoisture or in the presence of a catalyticamountof sodium hydroxide. The polyoxypropylene glycol product is thenreacted with ethylene .oxide to produce the ethylene oxide-propyleneoxide block copolymenl The polyoxyalkylene glycols and the esterden'vatives thereof contain from 2 to 3 carbon atoms per .alkylene oxideunit, An example of polyalkylene oxide 7 Percent Ammonium nitrate, atleast 70 Combustion catalyst consisting of a mixture of ammoniumdichromate and. insoluble Prussian blue 1.0 to 6.0 Cellulose acetate 6.0to 910 Ethylene glycol diglycolate 5.0 to 9.0 2,4-dinitrodipheny1 ether7.0 to 9.0 Diphenylamine gassing inhibitor 0.3 to 3.0 Finely dividedcarbon 0.3 to 3.0

Non-ionic surfactant material'selected from the a class consisting of'polyoxypropylene-polyoxy-" ethylene glycol having molecular weightswithin'the range of 2000 to about 3300, monoester derivatives. ofpolyoxyalkylene glycols having molecular weights within the range ofabout 1000 to about 2000 and at least one sorbitan oleate containing anaverage of from 1 to 3 oleyl radicals per mole of sorbitol intermediateused to obtain sorbitan 0.05 to 0.5

a position consists essentially, on a weight basis,'of: v2 Vcondensationproduct is the polyo xypropylenepolyoxy ethylene glycol,Pluronic L- 62. It corresponds to the general formula HO(C H O) ,(C H O),(C H O),,H and has a molecular weight of approximately 2000 of whichthe base unit, polypropylene oxide, provides from about 1500 to 1800 ofthe molecular weight. Pluronic L-62 is a liquid having a viscosity at 25'C. of 300 to 500 centipoises.

' -An example of an ester derivative of a polyoxyalkylene glycolproductv is Nonisol-250, the oleyl monoester of polyethylene oxide. Thefatty acid radicals of the monoester contain 12 to 20 carbon atoms. Itis a liquid having a molecular weight of about 1000.

The amount of surfactant material added to the composition is relativelysmall, i.e., usually within the range of 0.05 to 0.5% by weight of thecomposition, preferably 0.1% to 0.2%. 1 e I Thus to recapitulate, thepreferred gas-forming com- The composition of this invention can beprepared by several methods. The preferred procedure is as follows:

The binder is prepared by heating the mixture of plasticizer components,that is, ethylene glycol-diglycolic acid condensation product anddinitrodiphenyl ether to a temperature between about 120 C. and 150 C.The mixture is agitated while being maintained at an elevatedtemperature above the melting point of the mixture, agitation beingcontinued until a substantially homogeneous mixture has been obtained.To the homogeneous mixture is then added the cellulose acetate syntheticpolymer and the mixture is stirred to homogeneity to complete thefabrication of the binder. The desired amount of ammonium nitrate infinely powdered state is admixed with the desired amount of catalyst,also in finely powdered condition.

is continued until a homogeneous solid phase has been obtained andthe'mass is formed into shapes and configurations suitable for solidpropellant uses. The configurations are commonly called grains. Theseconfigurations or shapes may be formed by introducing the pasty massinto suitable molds and molding the material under pressure or shapesmay be made by extrusion. The compositions flow under 7 pressure attemperatures above C. and become dimensionally stable at temperaturesbelow about 90 C.

Burningrate test strips of the gas-forming compositions are prepared byextruding or molding the homogeneous composition at a temperature belowabout C. under a pressure of about 2.000 p.s.i., the test strips being.about '7 A" to about /3 in diameter and about 5" long. Ihe burning rateof these test strips is determined at series of pressures, i.e., 600,800, 1000, 1200, 1400, 1600 and 1800 pounds per square inchnitrogen'pressure in a Crawford bomb.

The burning rates in inches per second at the different pressures areplotted on log-log paper and this plot gives a straight-linerelationship, the burning rates being plotted as ordinates and thepressures being plotted as abscissa. The slope of this straight line isdefined as the pressure exponent or as the exponent of the burning rateasrelated to pressure in the formula I hite) wherein B is the linearburning rate at pressure p, [3 is the linear burning rate for thecomposition at 1000 p.s.i., p is pressure in p.s.i. in the burningchamber and n is the pressure exponent showing the dependence of theburning rate on pressure. Thus the exponent is the numerical value equalto the slope of the curve of burning rate in inches per second vs.pressure obtained by plotting 'the burning rate at various pressures onlog-log paper. Ammonium nitrate compositions usually have a pressureexponent of about 0.7 or higher. The lower the value of n, the less isthe detonating character of the decomposition of the gas-producingcomposition and the more even and smooth is the burning of thepropellant grain. Thus a sustained thrust rather than a detonation isproduced in the burning of the satisfactory propellant grain andsustained flow of gas from thegas generator is obtained if the pressureexponent of the composition is low. In general, pressure exponents lowerthan about 0.7 are desired.

One of the specifications for an acceptable solid propellant grain issatisfactory ballistic performance following prolonged storage at 77 C.The chief object of this invention is to overcomethe effects ofprolonged high temperature storage relative to detonation. This isaccomplished by incorporation in the grain of a small amount of thesurfactant in the grain composition, which also contains thediphenylamine.

Gas-producing grains are prepared from the pasty compositions containingthese ammonium nitrate combustion catalysts and binder by molding thecomposition into cylindrical grains under pressure of about 2000 to 4000psi. The size and shape of the grains are dependent upon their intendeduse. In one application grains for airplane assist take-E service areusually about 30" in length by 3" to 6 in diameter. These are providedwith centrally located holes of different shapes, that is, star form,cruciform, circular, etc. Test grains which were motor were 2.75" indiameter and 4 /2" to about in length and were provided with a starformcentrally located hole. These test data obtained by burning such grainsin the test motor indicate the thrust or impulse, uniformity of theburning rate and over-all performance of the compositions when used inassist take-off operations. For these operations, the grains are mountedin a conventional case and are ignited or fired by electrical or otherknown means. The temperature of the gases produced by firing of thegrain may be of the order of 1500 F. to 3000 F. and the pressure orimpulse thrust by the hot gases will be dependent upon the grain size,diameter of the nozzle and other factors. The gas-producing grainmaterial may be molded into disc form, stacks of discs being used asgas-forming propellant material for missile rockets.

Test grains of 2.75" in diameter by 5" in length are given a thermalshock test, which is referred to herein as thermal cycling test. In thistest, several of the grains of the given composition are subjected, inan oven, to a temperature of 175 F. for a period of two hours, followingwhich the grains are immediately subjected for a period of two hours toa temperature of F. to complete one cycle. Immediately following thisperiod the grains are again subjected to the 175 F. temperature .foranother period of two hours and then to a second cold temperaturetreatment at 75 F. for two hours, after which at least one of the grainsis allowed to come to room temperature, i.e., about 75 F. to F. This orthese grains are examined for resistance to deformation and shattering.Part of the grains are tested for firing qualities, the temperature ofthe grain being -75 F., 175 F. and normal ambient temperatures followingthe cycle treatment. The grains of this invention pass these cycletests, ignite at low, high and normal ambient temperatures following thecycle test and do not shatter. They burn uniformly following the thermalcycle treatment.

Another test to which the finished grains are subjected is that of hotaging. In this test, a number of grains are maintained at a temperatureof 170 F. in an oven in the presence of circulating air. Grains arewithdrawn periodically which are tested with respect to firing andburning qualities. In order to pass this test, the grains must beunaffected with respect to firing qualities. It must burn uniformly andnot detonate after being subjected to the high temperatures for a periodof 30 days. The addition of the sorbitan oleate, particularly thesorbitan sesquioleate, known commercially as Arlacel C, to the graincomposition causes a grain'to pass this 30-day hot aging test, asindicated hereinbelow. Like results are obtained in this test by theincorporation of Span 85, Pluronic L62 and Nonisol-25O.

Another requirement of the gas-forming compositions of this invention isthat of chemical stability at relatively high temperatures as indicatedby the substantial absence of gassing of the composition. The gassingtendency is measured by an arbitrary test which has been made moresevere with respect to temperature than would be imposed on thecomposition at any atmospheric temperatures.

This testis commonly designated as the C. Gas Stability Test and iscarried out as follows:

Three grams of a finely divided ammonium nitrate base composition isplaced in a vessel. The vessel is connected by tubing to a mercurymanometer system which is calibrated so that readings are in units ofvolume. The vessel is inserted to an opening in a metal block; thismetal block is provided with electrical heating elements and controlswhich permit the block to be maintained at a temperature of 135 C. Aperiod of 15 minutes is allowed for thesample to come to 135 C. At thistime, the manometer is set to zero. The gas rate in cc./g./hr. aftereach 15-minute interval is then plotted against the time of heating ofthe sample.

In general, it is considered that a composition which has an essentiallyzero gassing rate during the first hour of heating will be substantiallyfree of gassing tendency in storage at atmospheric temperatures. It isconsidered that a composition which has a gassing tendency not in excessof about 2 cc./g./hr. after the first hour of heating at a temperatureof 135 C. is satisfactory with respect to hot storage i.e., temperaturesas high as 77 C.

The effect on hot storage stability of including in the gas-formingcomposition a small amount of sorbitan sesquioleate is illustrated bythe comparison of the burning properties of grains prepared in Examples1 and 2 below.

Example 1 diglycolate plasticizer component was prepared by congem-pettiensing ethylene glycol with diglycolic acid in a mol ratio of glycolto acid of 1.2. The polyesterification reaction product had a specificgravity of 1.35, a'refraction index of 1.475, and had a molecular weightwithin the range of from about 300 to 400. The mixture of celluloseacetate and two component plasticizer was heated and stirred to obtain ahomogeneous mixture and to this mixture was then added 2.02 parts byWeight of diphenylamine and 0.10 part by weight of Arlacel C (sorbitansesquioleate). To this mixture was then added 73.9 parts by weight offinely ground ammonium nitrate with which was thoroughly mixed 0.5 partby weight of carbon black (ash-free), 1 part by weight of insolublePrussian blue and 2 parts by weight of commercial grade ammoniumdichromate. The ammonium nitrate was Special nitrate, 70% of which hadbeen ground at 14,000 r.p.m., thus giving an ammonium nitrate product,atleast 50% of which pass through a #100 US. Standard sieve. Thismixture was stirred and milled at temperatures with' in the range of 110to 120 C. for'a,fperiod of several hours until a homogeneous product wasobtained.

A part of the grain composition prepared in Experiment 1 was molded at2000 pounds pressureinto burning rate test strips, A" x A by 5" inlength. The burning rate of this-composition at 1000 p.s.i. was 0.115in./sec and the pressure component was 0.54. A small amount of-thegas-producing composition was tested in the 135 C. Gas Stability Test.No gassing was observed for two hours. The remainder of thegas-producingcomposition produced in Experiment 1 was molded at4000= pounds pressureto form cylindrical grains, each grain being provided with a starformhole throughout the length of the grains. These grains were about 2.75"in diameter and about 5". in length and hada density of about 1.5 8grams per cubic centimeter. The grain material molded very well. Severalof the grains were cycled through high 175 F.) and low (-75". F.)thermalshock treatment. Followingsuch treatment, the grains ignited andburned satisfactorily in the rocket motor at grain temperatures of 175F., -75 F. andat 70 F.

A number of the grains produced from, material of Example 1 weresubjected to the hot aging test, i.e., storage at 77C. (170 F.) overanextended period of time. It was found that these grains could be storedat 170 F. for more than 80 days without afiecting the burning propertiesof the grains, that is, grains stored for one day up. to more than 85days burned uniformly upon ignition in a test motor. The weight lossupon storage of'these grains, which weighed about 600 grams each, wasless than about 0.5% even after storage for more than 80 days at 170 F.

Example 2 In the second example gas-forming grain material wascompounded according to the same procedure as that. outlined in Example1', except that the composition contained no surfactant as definedhereinabove. The composition of this grain material compounded inExample 2'was as follows:

t Percent Cellulose acetate LL-l grade 1 6.90 2,4-dinitrodiphenyl ether8.05 Glycol diglycolate 5.98 'Diphenylamine 1 2.07 Insoluble Prussianblue 1.00 Ammonium dichromate 200 Carbon black 0.50

Ammonium nitrate 73.50

, duced in Example 2 above was the sameas the composition';ofthegas-forming material produced in Example 1 except that no sorbitansesquioleate surfactant was present in the composition. Although thematerial of Example 2 which contained diphenylamine showed no gassingduring a 1% hour heating of a 3 gram sample at C., grains of thismaterial which were stored'at F. malperformed after 7 days of the hotstorage. The grain wormholed and the resulting high pressure developedon 'firing ruptured a blowout disc. The burning rate of a strand of thisgrain propellant composition, at 1000 pounds pressure was 0.14 inch persecond with a pressure exponent of 0.74. Although the above compositionhasbeen designated as Example 2, it is to be understood that thecomposition is a control composition containing no surfactant and thehot storage test shows the characteristics of the propellant grain whenthe surfactant is not present in the composition.

' Example :3

p In .a third example, a gas-forming grain was compounded according tothe procedure outlined in Example 1 except that a' larger proportion ofcarbon was used in the grain'and Span 85, sorbitan trioleate, wasincluded in the grain composition in an amount of 0.1% by weight Twoadditional grain formulations were compounded according to the proceduredescribed in Example 1 to correspond to the composition of the basecontrol grain Example 2 except that in one formulation 0.1% ofNonisol-250 was included as a non-ionic surfactant and in the otherformulation 0.1% of Pluronic' L-62 was used as the non-ionic surfactant.Gas-forming propellant grains fabricated from these formulations passedthe 30-day hot aging test, that is, they were fired successfully afterbeing subjected to a temperature of 77 C. i.e., 170 F., for a period of30 days.

The drawings show a particular application of this invention to anassisted take-ofi unit. The ATO unit illustrated is designed to behung-under the wing of an aircraft; normally at least two units, i.e.,one under each wing, will be used. In Fig. l, the body of the unit ismade up of a tubular member 11 which is closed at one end and Whichisprovided with threads at the open end. Member 11 is provided with twoloops, 12 and 13. These loops are used to hang the unit from a carrier,not shown, which is attached to the wing of the aircraft. This carriermakes it possible to jettison the unit after take-off. A somewhat funnelshaped member 14 is attached to member 11 by engagement of the threadsat the large open end of member 14 with the threads on member 11. Member14 is provided with a. nozzle 16 through which thedecomposition'products pass. The size of nozzle '16 determines in partthe pressure maintained inside of the chamber formed by members 11 and14.

The solid propellant =fills the cylindrical portion of member 11. Thesolid propellant in this illustration consists of seven tubular grains,17, 18, .19, 20, 21, 22 and 23; each having an OD. of about 3" andhaving a centrally located cylindrical opening 1" in diameter, the fulllength of the grain; the grains are approximately 30" long. The grainsused herein contain about 2% of ammonium dichromate catalyst, 1% ofinsoluble Prussian blue catalyst, about 2% diphenylamine, 20% of binderand 73.5% of ammonium nitrate. Each grain has the annular area at eachend inhibited against burning by a coating consisting of a mixture ofvistanex resin and carbon black in order that the burning may take placeon the cylindrical surfaces only. For some uses it is desirable to havea grain which burns cigarette fashion in which case the outer surfaceand one end of the grain will be inhibited to prevent combustion.

Although a tubular grain is illustrated herein, the invention is notlimited to such a grain. Any particular shape may be utilized. Examplesof other shapes are cylinder, cruciform, triform, hexaform, octaform andslab. In the case of perforated grains, the perforation may be circularor star-shaped with various numbers of points in the star. Furthermore,a single cylindrical grain having one or more longitudinal perforationsmay be utilized in some cases, instead of the multigrain unit shown. a

The grains are held in longitudinal position and prevented by slidingback and forth in the combustion chamber by means of a wire grid 26.Wire grid 26 consists of a ring cut to fit the threads of member 14 andprovided with a grid work of metal wires that will resist the hightemperature existing in the combustion chamber.

On one side of the conical portion of member 14 there isprovided for thecombustion chamber a safety venting means 23. Venting means 28 comprisesa tubular member fastened to member 14, which tubular member has fullaccess to the combustion chamber and is provided with a rupture disc,not shown. The rupture disc is of such construction that excess pressurein the combustion chamber will blow out the disc, whereby the pressurein the combustionchamber will be held below the point of serious damageto the unit.

An igiiter means is positioned with-in member 14 so as to close off thenozzle 16. The igniter means consists of a container 31 filled withblack powder, or some other easily ignited material, which can produce alarge volume of gases at elevated pressure. A squib 32 'for igniting thepowder is attached to the container 31 in communication with the powdercontained therein. Electrical wires 33 connect a wire in the squib tothe electrical system of the aircraft and a switch therein (theconnections to the aircraft are not shown).

The ATO unit, is assembled as follows: the grains are inserted intomember 11. Venting means 28 are fastened to member 14. Igniter 31 isinserted through the large open end and fitted so as to close thenozzle, the wires 33 having first been passed through the nozzle 16. Thewire grid 26 is screwed into the large open end of member 14. Theassembled nozzle portion is then securely screwed onto member 11.

The assembled unit is then attached to the wing of the aircraft by loops12 and 13; wires 33 are connected to the electrical operating assemblyin the aircraft. When the pilot desires to obtain the assisted take-off,he throws the switch which causes the current to pass through wire 33and to heat up the firing wire in squib 32, which in turn ignites thepowder in the container 31.

The container is of sufficient strength to withstand the initialpressure generated by the gases from the powder. The hot gases raise thepressure in the combustion chamber high enough to permit the grain toignite. The combustion of the grain causes the pressure in the chamberto rise to a point which cannot be resisted by container 31. Thecontainer disintegrates and the pieces are discharged through nozzle 16.The total time from throwing the switch to full operation of the unit ison the order of less than 0.5 second.

As the gases pass out of the nozzle the reaction acts on the aircraftand adds its thrust to assist the aircrafts propeller; a marked increasein forward speed results and permits the aircraft to take off in ashorter space of time or it permits lifting a load heavier than could beairborne by the use of the propellers alone.

The conventional placement of the igniter may be used with the lowercatalyst content grains. However, it is necessary to use a much heavierpowder charge in the igniter or, preferably, the nozzle is provided witha rupture disc, which is set to blow out at about 500 psi. Other methodsof igniting the grain can be readily devised.

While the utility of the shaped grain composition has been illustratedby means of an assisted take-off operation, it must be understood thatthe solid propellant of this invention can also be used for otherpurposes. Some of these are air-to-air missiles, air-to-ground missiles,ground-to-ground missiles, etc. An important use of the invention liesin the production of gases at elevated pressures in a stationary or aportable system; discontinuous operation is readily obtained when usingabout 2% of catalyst as the composition can be extinguished readily bymerely depressuring the combustion chamber.

Having thus described this invention, what is claimed 1. A gas-formingcomposition consisting essentially of at least about 70% by weight ofammonium nitrate; from about 1% to about 6% by weight of an inorganiccombustion catalyst consisting essentially of a mixture of ammoniumdichromate and insoluble Prussian blue; from about 18% to about 27% byweight of a binder material, which bindermaterialconsists essentially offrom about 18 to about 35% by weight of cellulose acetate, whichanalyzes between about 51 and 57% by weight of acetic acid, from about20 to about 40% by weight of a liquid polyester condensation product ofethylene glycol with diglycolic acid, said polyester condensationproduct having a molecular weight within the range of from about 250 toabout 600, said binder material also containing from about 20% to about40% of a nitrodiphenyl ether based on the total weight of the binder;from about .3 to about 3% of diphenylamine as a gassing inhibitor, basedon the total weight of a gas-forming composition; from about .3 to about3% byweight of finely divided carbon and from about 0.05 to about 0.5%by weight of a nonionic surfactant material selected from the classconsisting of polyoxypropylene-polyoxyethylene glycols having molecularweights within the range of 2000 to about 3300, poiyoxyalkylene glycolfatty acid monesters having molecular weights of about 1000 to about2000, the fatty acid radicals of which contain 12 to 20 carbon atoms andat least one oleyl ester of mono'anhydro sorbitol.

2. The gas-forming composition described in claim 1 wherein thenitrodiphenyl ether is 2,4-dinitrodiphenyl ether.

3. The composition of claim 1 wherein the finely divided carbon iscarbon black.

4. The gas-forming composition as described in claim 1 wherein the oleylester of anhydro sorbitol is sorbitan sesquioleate.

5. The gas-forming composition as described in claim 1 wherein the oleylester of the anhydro sorbitol is a sorbitan trioleate.

6. The gas-forming composition as described in claim 1 wherein thenon-ionic surfactant material is a polyoxypropylene-polyoxyethyleneglycol having a molecular weight of about 2000.

7. The gas-forming composition as described in claim 1 wherein thenon-ionic surfactant material is a polyoxyalkylene glycol-fatty acidester having a molecular Weight of about 1000.

8. A shaped grain propellant composition consisting essentially on aweight basis of about 73.9% ammonium nitrate; about 6.8% of celluloseacetate, which analyzes between about 54 and 56% by weight of aceticacid, about 7.9% of 2,4-dinitrodiphenyl ether; about 5.9% of thecondensation product of ethylene glycol with digly- $942,964 l3 7 l4colic acid; about 2% of diphenylamina; about 1% of References Cited inthe file of this patent l I msolub c Prusslan blue and about 2% ofammomum d1 UNITED STATES PATENTS chromate catalyst; about 0.1% ofsorbitan sesquioleate having a specific gravity at 25 C. of 0.95-1.00and about 2,159,234 Taylor May 23, 1939 0.5% of a carbon black. 52,628,561 Sage et a1. Feb. 17, 1953

1. A GAS-FORMING COMPOSITION CONSISTING ESSENTIALLY OF AT LEAST ABOUT70% BY WEIGHT OF AMMONIUM NITRATE, FROM ABOUT 1% TO ABOUT 6% BY WEIGHTOF AN INORGANIC CONBUSTION CATALYST CONSISTING ESSENTIALLY OF A MIXTUREOF AMMONIUM DICHROMATE AND INSOLUBLE PRUSSIAN BLUE, FROM ABOUT 18% TOABOUT 27% BY WEIGHT OF A BINDER MATERIAL, WHICH BINDER MATERIAL CONSISTSESSENTIALLY OF FROM ABOUT 18 TO ABOUT 35% BY WEIGHT OF CELLULOSEACETATE, WHICH ANALYZES BETWEEN ABOUT 51 TO 57% BY WEIGHT OF ACETICACID, FROM ABOUT 20 TO ABOUT 40% BY WEIGHT OF A LIQUID POLYESTERCONDENSATION PRODUCT OF ETHYLENE GLYCOL WITH DIGLYCOLIC ACID, SAIDPOLYESTER CONDENSATION PRODUCT HAVING A MOLECULAR WEIGHT WITHIN THERANGE OF FROM ABOUT 250 TO ABOUT 600, SAID BINDER MATERIAL ALSOCONTAINING FROM ABOUT 20% TO ABOUT 40% OF A NITRODIPHENYL ETHER BASED ONTHE TOTAL WEIGHT OF THE BINDER, FROM ABOUT .3 TO ABOUT 3% OFDIPHENYLAMINE AS A GASSING INHIBITOR, BASED ON THE TOTAL WEIGHT OF AGAS-FORMING COMPOSITION, FROM ABOUT .3 TO ABOUT 3% BY WEIGHT OF FINELYDIVIDED CARBON AND FROM ABOUT 0.05 TO ABOUT 0.05% BY WEIGHT OF ANONIONIC SURFACTANT MATERIAL SELECTED FROM THE CLASS CONSISTING OFPOLYOXYPROPYLENE-POLYOXYETHYLENE GLYCOLS HAVING MOLECULAR WEIGHTS WITHINTHE RANGE OF 2000 TO ABOUT 3300, POLYOXYALKYLENE GLYCOL-FATTY ACIDMONESTERS HAVING MOLECULAR WEIGHTS WITHIN THE RANGE OF 2000 TO ABOUTACID RADICALS OF WHICH CONTAIN 12 TO 20 CARBON ATOMS AND AT LEAST ONEOLEYL ESTER OF MONOANHYDRO SORBITOL.